Aims Catheter ablation is underutilized in atrial septal defect (ASD) patients who have undergone implantation of an atrial septal occluder (ASO). This study evaluates the feasibility and safety of catheter ablation of atrial fibrillation (AF) in this subset of patients. Methods and results Sixteen patients (age 56 +/- 12 years, 10 men) with drug-refractory AF (10 paroxysmal and 6 persistent) and previously implanted ASO were enrolled. Balloon dilatation of the closure device was performed if the native septum passage could not be achieved. For paroxysmal AF, the ablation strategy was circumferential pulmonary vein isolation (CPVI), and for persistent AF, additional linear ablation was performed. Transseptal access was achieved through the native septum in 11 patients (Group A) and through the ASO using balloon dilatation in 5 patients (Group B). Circumferential pulmonary vein isolation was achieved in all 16 patients, and linear block was achieved in all persistent patients except for 1 patient who did not achieve mitral isthmus block. The transseptal, total fluoroscopy, and procedural durations were 5 +/- 3 vs. 38 +/- 8 min, 31 +/- 11 vs. 54 +/- 15 min, and 165 +/- 35 vs. 224 +/- 36 min, respectively, in Group A vs. Group B, respectively (all P < 0.05). No shunt at atrial level was detected by transthoracic echocardiography at 3-month follow-up. During a follow-up of 16 +/- 6 months, sinus rhythm was maintained in 12 of 16 patients. No severe complications were observed. Conclusion In ASD patients with ASO, catheter ablation of AF is feasible, safe, and effective. The balloon dilatation technique can facilitate transseptal access through the ASO.